This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The establishment of human immunodeficiency virus type 1 (HIV-1) and HIV-2 infections in humans is the consequence of multiple cross-species transmissions of simian immunodeficiency viruses (SIVs) from chimpanzees (SIVcpz) and sooty mangabeys (SIVsmm), respectively. While over 35 African nonhuman primate (NHP) species carry their unique strain of SIV, many of which resulted from simian-to-simian transmissions, these natural SIV infections are chronic and productive but not associated with immunodeficiency. Baboons (Papio hamadryas ssp.), one of the most widely distributed African NHP species, do not seem to be naturally infected with SIV even though their home-ranges overlap those of many SIV-infected NHP species. Despite serological evidence for SIV infection in very few wild-living baboons, isolation of a replicating virus from these animals has not been reported. The reasons for this apparent natural resistance to SIV infection are not known. Baboons have been shown not to be resistant to experimental infections with HIV-1 and can be infected with HIV-2 and SHIVs. Further, in our hands, baboon lymphocytes support productive infection with SIVmac and SIVagm, indicating that intracellular antiviral factors like APOBEC3 proteins, Trim-5[unreadable], and Tetherin probably do not contribute to this resistance. Our long-term goal is to identify the factors responsible for natural resistance to SIV infection in baboons.